A projection lens having a long back focus is conventionally required in color projectors that employ liquid crystal display panels which modulate differently colored beams of light with image information for that color, so that a beam-combining prism may be inserted between the lens and the display panels in order to compose the color information into a single, full-color display that is then projected as full-color image onto a projection screen. In order to provide a large projected image using a short projection distance, it is necessary to widen the angle of view of the projection lens. Moreover, a low distortion lens is required in order to project images, which have been formed using liquid crystal display panels having an aperture shaped as a rectangle, without pin-cushion or barrel distortion noticeably affecting the rectangular outline of the projected light.
A retro-focus lens having a wide angle of view and a long back focus is known and has been used as a wide-angle lens for single-lens reflex cameras. However, such a retro-focus lens has a large exit angle and its luninous flux is not telecentric. Thus, in using such a lens with a liquid crystal projector there is a problem in that images projected onto a projection screen have irregular colors.
In order to solve such problems, the applicant of the present invention has previously disclosed, in Japanese Laid-Open Patent Application H09-096759, a projection lens that has a long back focus, forms a luminous flux with sufficient telecentric properties so as to avoid irregular colors when used with a liquid crystal projector, and is well-corrected for distortion.
As the number of pixels in liquid crystal display panels being sold has tended to increase with each passing year of technological advancement, the size of each pixel has tended to decrease, and this creates a need for projection lenses which have higher resolution. Therefore, it is desired to provide a projection lens having higher resolution and better correction of chromatic aberrations and distortion than provided by the lens disclosed in the above-noted laid-open patent application.